The present invention relates to a reforming apparatus of the type comprising an indirect heat exchange zone for the reforming reaction of a gaseous flow comprising methane and steam into CO, CO2 and H2.
In the description given below and in the following claims, the term: xe2x80x9cmethanexe2x80x9d is generally understood to mean a raw material which is a source of hydrogen and carbon such as e.g. methane itself or a mixture of liquid and/or gaseous hydrocarbons such as natural gas and naphtha.
As known, in the field of methane reforming to obtain hydrogen and carbon which are indispensable for the synthesis of products such as ammonia and/or methanol, the requirement to make available an apparatus which on the one hand allows obtaining a reforming reaction of the methane as complete as possible and on the other hand requires low energy consumption and investment and maintenance costs and is easy to implement is ever more pressing.
To satisfy the above mentioned requirement, an exchanger-type reforming apparatus, i.e. having a heat exchange zone for the methane reforming reaction, has been proposed in the industry.
In this apparatus, the high quantity of heat necessary for the endothermic reforming reaction is supplied by indirect heat exchange with a flow of heating gas fed to such apparatus.
In particular, in ammonia plants where the methane reforming reaction is performed in two distinct sections called primary and secondary reforming with the latter operating at a higher temperature than the former, it is possible to utilize the hot reacted gas coming from the secondary reforming section as a heat source for the primary reforming section.
The exchanger type reforming apparatus is generally used in the state of the art in ammonia, methane or hydrogen synthesis processes to replace the conventional primary reformer, as described for example in EP-A-0 298 525.
Although advantageous in many ways the above described apparatus displays a series of drawbacks the first of which is being of very complex construction requiring high investment costs.
Indeed, this apparatus comprises in it a plurality of bayonet-type tubes, i.e. consisting of an external tubular element with blind end for indirect heat exchange between the heating gas flow and the gaseous reagents (methane and steam), and an internal tube for extraction of the reaction products.
As may be readily imagined, a structure of this type is complex and costly to construct, difficult to access for maintenance operations, and involves large-diameter reforming apparatus.
In addition, since the reforming reaction is of the catalytic type, it is necessary that the annular space defined between the external tubular element and the internal tube is filled uniformly with catalyst and that the catalyst is replaced periodically. These operations are clearly hindered or at least made difficult by the presence of the internal tube.
Lastly, the use of bayonet-type tubes displays disadvantages even from the energy viewpoint, because there is significant undesired heat exchange between the reacted gas flow and the reacting gas flow, with the added risk of occurrence of metal dusting corrosion of the internal tube due to the reacted gas if the latter is cooled excessively.
JP-A-4154601 describes a reforming apparatus of the exchanger type comprising a plurality of individual tubes filled with catalyst and outside which flows the heating gas.
The tubes are affixed at their ends to respective tube plates which are also appropriately affixed to the reforming apparatus.
Although simpler to construct and operate than the bayonet tubes the heat exchange tubes described in JP-A-4154601 display the serious disadvantage that they are not free to expand if subjected to high temperaturesxe2x80x94as in the case of the reforming reactionxe2x80x94with the risk of cracking or even breakage thereof and thus mixing of the reacting gas with the heating gas and damage to the apparatus.
It follows that this type of apparatus not only entails high maintenance costs for replacement of defective tubes, but is not able to ensure optimal and reliable long term operation.
Because of these disadvantages, the exchange-type reforming apparatus according to the prior art has heretofore found little application despite the ever more urgently felt requirement in the industry.
The problem underlying the present invention is to make available a reforming apparatus which would be simple to implement, reliable, and would provide a methane reforming reaction as complete as possible with low investment, operating and maintenance costs as well as low energy consumption.
The above mentioned problem is solved according to the present invention by a reforming apparatus for the conversion of methane and steam into CO, CO2 and H2 of the type comprising:
a substantially cylindrical external shell in which are defined an indirect heat exchange zone and a zone for feeding a gaseous flow comprising methane and steam to the indirect heat exchange zone;
an opening formed in said shell for feeding in said indirect heat exchange zone a heating gas flow as heat source for said conversion; and which is characterized in that it also comprises:
a plurality of floating-head tubes containing a reforming catalyst, extending longitudinally in said indirect heat exchange zone and in fluid communication with said feeding zone;
a chamber for collecting a gaseous flow comprising CO, CO2, and H2 obtained from said conversion and positioned downstream of said tubes;
a duct open in said collection chamber for extracting from the shell said gaseous flow comprising CO, CO2, and H2.
In the description given below and in the following claims, the term: xe2x80x9cfloating-head tubesxe2x80x9d is understood to mean tubes having at least one end (head) structurally free to move (floating) to allow heat expansion of the tubes.
Advantageously, the reforming apparatus according to the present invention calls for a collection chamber for the reacted gas in fluid communication with a plurality of tubes containing catalyst for indirect heat exchange, and a duct for extraction of this gas from the shell.
In this manner, all the gasxe2x80x94once the reforming reaction has taken placexe2x80x94is collected in the same chamber and extracted by means of a single duct.
Thanks to this particular structure, it is possible to obtain exchange-type reforming apparatus which is reliable, extremely simple to construct and has low implementation costs and which is at the same time effective as regards methane reforming reaction, without the drawbacks typical of the prior art apparatus.
In particular, maintenance operations and loading or replacing the catalyst in the tubes are facilitated by the presence of a plurality of individual floating-head tubes independent of one another.
In addition, since the reacted gas is all collected in a single chamber and extracted from the shell by means of a duct which is thermally independent of the heat exchange tubes, the undesired heat exchange between the reacted gas and the reacting gas is advantageously eliminated to avoid the danger of metal dusting corrosion of the extraction duct and to reduce operating costs as compared with the prior art apparatus.
According to a preferred embodiment of the apparatus in accordance with the present invention, the extraction duct is advantageously arranged coaxially with said shell and extending parallel to said tubes through the indirect heat exchange zone and the feeding zone, from the collection chamber to a gas outlet opening from the shell.
In this manner, there is obtained a very simple and compact structure, permitting at the same time effective compensation for the expansion of the different parts of the apparatus caused by the different thermal stress to which these parts are subject and by the use of different materials.
In particular, it is possible to appropriately and reliably compensate the different expansion rates to which the heat exchange tubes and the reacted gas extraction duct are subjected, without thereby having to give up extremely simple apparatus from the structural point of view.
Indeed, thanks to the special arrangement of the extraction duct there is advantageously obtained a collection chamber which is also of the floating type, with the heat exchange tubes and the extraction duct free to expand in mutually opposite directions with respect to the feeding zone.
In this manner, the different expansion rates of the materials not only do not create mechanical problems for the apparatus but can be mutually compensated in a certain manner.
Advantageously, according to this embodiment, between said duct and a tube plate positioned between said feeding zone and said heat exchange zone as between said duct and said shell, there are provided suitable gas sealing means so as to avoid undesired by-pass of the reaction gas or reacted gas and at the same time to permit the different heat expansion rates of the apparatus.
Thanks to the present invention, the gas sealing means which ensure correct operation of the apparatus are reduced to the minimum and concentrated between the extraction duct, the tube plate and the external shell only.
Preferably, the gas sealing means are arranged near said outlet opening so as to facilitate access to the gas sealing means and thus simplify and aid maintenance thereof.
The characteristics and advantages of the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.